Telephone system.



UNITED STATES PATENT oFFIoE.

CHARLES W. KECKLER, OF NEWARK, NEW JERSEY, ASSIGNOR, BY MESNE ASSIGN- MENTS, TO WESTERN ELECTRIC COMPANY, INCORPORATED, A CORPORATION OF NEW YORK.

TELEPHONE sYsTEm.

Specification of Letters Patent.

Patented Jan. 1, 1918.

To all whom it may concern.-

Be it known that I, CHARLES W. lKECK- LER, a citizen of theUnited States, residing at Newark, in'the county of Essex and State of New Jersey, have invented certain new and useful Improvements in Telephone Systems, of which the following is a full, clear, concise, and exact description.

This invention relates to telephone systems, and more particularly to house or intercommunicating systems.

The object of this invention is to provlde improved means for rendering secret telephone service, that is, to prevent other stations of a telephone system interfering with or overbearing a conversation carried on between two connected stations or a conversation carried on over a trunk circuit extending between the intercommunicating system and another telephone system exterior thereto.

One feature of this invention is to provide improved means for preventing other stations of a telephone system intercepting a call or interfering with the operations by which one station calls and establishes a dispense with an attendants station and to provide that trunk circult connections may be handled and transferred from one station to another by any station in the system. Still another feature of this invention is to provide improved means whereby one station of a secret service intercommunicating system, having connection with an outside line, may bring in another intercommunicating station on the system, and all three stations converse together without disturbing the secrecy thereof.

Inaccordance with the features of this invention, two relays are added to the usual -in rendering the calling position of the equipment, consisting of a telephone set, a call signal and a plurality of 'switching keys at each station; and a plurality of relays, a repeating coil and two call signals are provided for each trunk circuit extending from the mtercommunicating system to a telephone exchange. These relays render all calling keys, call signals, except one of the signals associated witha trunk circuit, and switching keys normally disabled.

' In this system, relay action takes place after each manual operation in the processes of establishing a connection between two. stations or between stations and a trunk c1rcu1t.

In originating a call, the first manual operatlon sets up relay action to exclude any other station which may seek connection with the calling station or with the trunk circuit when a call thereon originates at one of the intercommunicating stations. At the mtercommunicating stations this relay action also renders the calling position of the switching keys effective'to initially setup a second relay action which results, first,

switching keys effective as ringing keys, second, to complete the circuits between the telephone set and the switching keys, third, to establish a locking circuit for the last relay operated, and fourth, to cause relay action at the station called. The circuit changes, due to the operation of the relay at the latter station, first disable the circuit over which the second relay action at the calling station took place, thereby guarding the called station against another station wh1ch may attempt to secure a connection therewith and to render the calling signal effective to respond to the converted ringmg key at the calling station.

The switching keys are arrangedv to be locked by a common plate in-an intermediate position at which certain contacts are effective to complete a connection for talking. A second non-locking position of the switchmg keys is present when the plunger of a key is fully depressed andrenders other contacts efi'ective'to first complete a relay operating circuit and second to act as a ringing key.

In this system identical equipment is used at each station; Therefore, it will be un-? derstood that a description'bf the features The characteristic operating and service features of the present invention may be described as follows: Referring to the drawing, the matter of service between stations local to the intercommunicating portion of the system will first be considered. A calling signal 23 and the calling-position contacts 31 of a switching ke B at station A are normally disabled at t e separated contacts 12 of a relay 10, and a telephone set permanently connected with an associated pair of line conductors 15 and 16 is normally disconnected from the common or bus conductors 13 and 14 of the switching keys by the open position of the contacts 27 and 28 of the relay 25. In originating a call, the receiver is first removed from its switchhook, say at station A. Talking battery is thereupon supplied to the telephone set from the battery 1 through the two operating windings of the relay 10. The

operation of this relay follows andrenders the calling position of the switching keys effective, and the switching key B associated with the line of the desired. station, say the station B, is now fully depressed. The first impulse of current through the calling contacts 31 of the switching key B initially operates the rela 25 at this station.

This circuit is complete from the battery conductor 8, the contacts 12 of the relay 10, winding of the rela 25, contacts 31 of the key B, the line con uctor 33, the contacts 43 of a relay 42 at station B, thence to the battery conductor 7. After being initially operated, the relay 25 is self-locked locally at station A on the battery 3 over a circuit from battery conductor 8, the contacts 12 of relay 10, the winding and contacts 26 of the relay 25 and contacts 17 of the switchlocking plate 18, to the battery conductor 7. The contacts 27 and 28 of the relay 25 and the contacts 29 and 30 of the switch key B now include the telephone set at this station with the line conductors 33 and 34 respectively of the station B. In this position the left-hand winding of the relay 10 is connected in parallel over the line conductor 34 with the left-hand winding of the relay 42 at station B. The relay 42 is thereby operated and separation of its contacts 43 disconnects the clrcuit through which the rela 25 at station A was initially operate and the closing of the contacts 44 of the relay 42 prepares an operating circuit for the call signal 40. The closing of the contacts 26 of relay25 at station'A reverses the polarity of the calling battery 3 at the contacts 31 of the key 3, thereby rendering this key effective to operate the signal 40 at station B. This signal operating circuit includes the battery conductor 7, contacts 17 of the key-locking bar 18, contacts 26 of the relay 25, contacts 31 of the key B line conductor 33 to station B, thence over the switchhook contacts 36, the signal 40, contacts 44 of the relay 42 to the battery conductor 8. During the time the call bell 40 is active, the static or capacity balance of the telephone receiver circuits at station A change in unison with the operation of the contacts of the bell, and a corresponding tone is accordinglypr'oduced in the telephone receiver. Thls effect is possible for two reasons, first, that the signaling current is applied to the line conductor 33. without disconnecting this conductor from the telephoneset at station A, and second, that the condenser effect at the telephone receiver is augmented, due to the left-hand winding of the relay 42 at station B now forming a ath parallel with the left-hand winding 0 the relay 10 at station A, thereby dimlnishing the impedance between the telephone receiver and the capacity surface of the talking battery and circuits. The presence of this tone indication signifies to a calling party that the station wanted is idle, and conversel the absence of this tone indicates that t e station wanted is busy. A further service advantage of the tone indication is that on cessation of tone, the callin "party will know that the station called as answered.

The removal of the telephone receiver from its switchhook at station B, in response to the call, disconnects the signal bell 40 by the separation of the switchhook contacts 36, and the closing of contacts 37 and 38 of the switchhook completes a circuit throu h which talking current is supplied from t 0 battery 1, over the right and left-hand windin of the relay 42 to the telephone set.

uring the talking period, relays 10 and 25 at station A remain operated while onl the relay 42 at the station B is energize and so long as the receiver at either one of the connected stations is retained off the switchhook, the double wound relay at the station first to replace the receiver on the switchhook is retained by its left-hand winding, as this winding is then established in parallel with the left-hand winding of the corresponding relay at the other'stationL This action is reciprocal withrespec't to two connected stations, and is the same as the corresponding operation already described in c lmnection with the processes of making a ca 1.

At the conclusion of a conversation, and whenthe telephone receiver at each station has been replaced on its switchhook, all of the relays are deenergized and the circuits restored to the normal condition. The restoring of the circuits to a normal condition, described in connection with-the equipment at station A, is due, first, to the denergizing of the relay 10 on cessation of talking current through the telephone set, and second, the separation of the contact 12 of this relay opens-the locking circuit of the relay 25,

which thereafter remains inert due to the In order to describe the conditions which prevent other stations on the system interfering with or securing a connection with either one of two connected stations, let-itbe assumed that the station A is a third station and attempts to secure a connection with the station B while the latter station is busy. The relay 10 responds to the removal ofthe receiver from the switchhook at station A and the key B is next fully depressed. The circuit through which the relay 25 now attempts to operate may be traced from the battery conductor 8, contacts 12 of the relay 10, winding of the relay 25, contacts 31 of the key B line conductor 33 to the station B, thence to a. point where this circuit stands open at the separated contacts 43.0f the relay 42. Failure of the relay 25 to operate allows'its contacts 27 and 28, respectively, to remain separated, and the telephone set at station A is thereby excluded from'the line circuits of the station B.

From the foregoing description it will be evident that the separation of the contacts 43 of the relay 42 at the station B, and corresponding contacts of similar relays at other stations, constitute the means by which other stations are excluded during the time calling processesare in operation of being transmitted or received, as well as during the time two lines are connected for conversation.

- A further operating feature of this system relates to an alternate method of'restoring or denergizing one of the relays at a depressed, the locking bar 18 will nection with another station, without first replacing the receiver on the switchhook, anot a circuit of the relay 25, permitting this relay to release. This feature renders it impos-' sible for the relays 25 to first be operated over an idle line, and then by operating a key corresponding with another line on which a party may desire to listen, succeed in completing an irregular connection.

In describing the operations taking place when the trunk circuit C is used, the processes by which a station-say the station A-secures a connection with a subscriber whose line may be assumed as terminating in a telephone exchange (not shown) to which the trunk circuit (3 extends, will now be described.

The necessary manual operations at the station A correspond with those already described for a local call. The receiver is removed from the switchhook and a key C corresponding with the trunk circuit C, is then fully depressed. The relay 10, now energized by the talking current through the telephone set, permits the key C to complete a circuit from the battery conductor 8 of the calling battery 3, contacts 12 of the relay 10, winding of the relay 25, contacts 45 of the key C, a conductor 48, to the trunk circuit equipment, thence over the contacts 61 of a relay 60, the contacts 55 of a relay 54, to the battery conductor 7. The current impulse through the circuit just traced initially operates the relay 25 at station A,'after which it is locked,-as previously described. The telephone set is now connected through the contacts 27 and 28 of the relay 25, the bus conductors 13 and 14 and contacts 46 and 47, respectively, of the key 0 in bridge on the trunk conductors 49 and 50. A branch circuit isthereby established from the conductor 5 of the battery 1, the winding of a relay 51, one winding of a repeating coil 75 to the conductor 49, thence through the telephone set at station A, returning over the conductor 50, a second winding of the repeating coil 75, to'the conductor 4 of the battery 1. The operation of the relay 51 now eflects the operation of a relay 54 and a relay '58 over the following circuits: conductor 8 of the battery 3, contacts 53 of the relay 51, the winding of the relay 54 to the conductor 6 of the battery 2,

which is connected in accumulative series with the battery 3. The second circuit extends from the battery conductor 8, the contacts 53 of the relay 51, the left-hand winding of the relay 58 to the battery conductor 6. The separation of the contacts 55 of the relay 54 disconnects the battery conductor 7, over which the relay 25 at station A was initially operated, and the contacts 56 of the relay 54 now complete a circuit for the signal 66. At the station A the energized position of the relay 25 connects the neutral conductor 7 of the serially connected batteries 2 and 3 by way of the contacts 17 of the key-lockingplate 18 to the contacts of the key 0 wh ch 1s continued fully depressed; thence, this circuit continues over the conductor 48 to the trunk circuit signal 66, the contacts 56 of the relay 54 to the battery conductor 8. The operation of this signal, which is of the v1- brating-interrupter type, sets up changes in.

the static or capacity balance of the circuit of the telephone receiver at the station to produce a tone indication 'in the receiver. The signal 66 maybe located at any point where it may be desirable to keep a record of the outgoing trunk circuit calls. Although the contacts 59 of the relay 58 complete a circuit to establish a rela 60 in series with the main conductors 0 the trunk circuit to the central-energy exchange (not shown), this relay does not operate, owing to the limited quantity of current permitted by the comparatively high resistance of a signal control relay at the exchange. This signal relay, however, is operated to display a call signal at the exchange. On the call being answered at the exchange, the low resistance of the link circuit permits a suflicient quantity of current from the exchange battery to operate the relay 60. The separation of the contacts 61 of the relay 6O disconnects the signal .66, and the calling party at station A may now release the key C to its locking position and talk, by way of the conductors 49 and and the repeating coil 75, with the exchange. 'In' the talking position the relays 10 and 25 at station A and the relays 51, 54, 58 and 60 of the trunk circuit C remain energized.

Telephone service of this character usually requires that the connection be retained at the exchange until a disconnect signal'is received from the station of the intercommunicating system, and in the present system this takes place as follows: On disconnecting the telephone set from the bridged connection over the trunk conductors 49 and 50, by replacing the receiver on the switchhook at station A, the relay 51 is released, and this relay in turn releases the relays 54 and 58. The separation of the contacts 59 0f the relay 58 disconnects the relay 60, thereby causing a cessation of battery current over the line of the trunk circuit, and accordingly a disconnect signal is displayed at the telephone exchange.

Innnison with the releasing of the relay 51 of the trunk circuit, the telephone set at station A also disconnects the relay 10, which in turn opens the locking circuit to viating the necessity of a so-called attendant station which is usually required for handling this service. The signal 70, connected in series with a condenser, thence in bridge on the main line conductors of the trunk circuit responds to ringing current applied to the trunk circuit when a calloriginates at the exchange. The signal 70, or signals auxiliary thereto, may be so located that several of the stations will receive the indication of an incoming trunk circuit call, thereby insuring that these calls will receive better attention. i

In describing the processes by which an incoming trunk call may be answered and transferred to another station, let it' be assumed that in response to the operation of the si al 70 the call is answered by the station but the party wanted is at the station B. At the station A the receiver is first removed from the'switchhook and, secondly, the key 0 is fully depressed, thereby energizing the relays 10 and 25 at station A, and the relays 51, 54, 58 and 60 of the trunk circuit, all in the same manner and for the purposes already described in the matter of originating a trunk circuit call from the station A. At this time, however, the relay 60 receives suflicient current from, the link circuit at the distant exchangeto effect its optrunk circuit. On ascertaining that the call is for a party at the station B, a transfer keyC at the station A is momentarily depressed and completes a circuit from the neutral battery conductor 7, the contacts 17 of the key-locking plate 18, contacts 26 of the relay 25, contacts 69 of the key 0*, conductor 71, the left-hand winding ofthe relay 67, the contacts 64 of the relay 63, to the battery conductor 6. This momentary inclusion of the relay 67 on the battery 2 initially operates the relay 67, which is then self-locked on the series circuit formed of the batteries 2 and 3, over the following cir-,

cuit: battery, conductor 6, contact 64 of the relay 63, the right-hand winding and the contacts 68 of the relay 67, contacts 62 of the relay 60, the right-hand winding of the relay 58 to the battery conductor 8. At the *of the trunk key station A the telephone receiver isretained off the switchhoo and the key B corres ondin with the line conductors to station is ful depressed. In depressing the key B, a Well-known action of the locking plate 18 releases the trunk circuit key C and momentarily opens the contacts 17 thereby deenergizing the rela 25. On the restoring 1 and the relay 25, the

telephone set at station A is disconnected from the trunk circuit conductors 49 and 50,

thereby permitting the relay 51 to release, and in turn this relay releases therelay 54, the relay 58 remaining energized over the circuit ust traced through its right-hand Winding. If, now, the station B is idle, the relay 25 at station A will be operated, and a connection established between these stations, in a manner already described. On the party at station B receivingthe request to take up the trunk circuit call, the telephone receiver is retained oif the switchhook and the ke O is depressed to the intermediate or ocking position, and the transfer key C is depressed momentarily. The key C completes a circuit from the battery conductor 8, contacts 44 of the relay 42, the winding and contacts 39 of the relay 35, the contacts 41 of the key C conductor 71, the winding of the relay 63, contacts 52 of the relay 51, to the battery conductor 6, thereby operating the relay 35 at station B and the relay 63 of the trunk circuit. The relay 35 is rendered self-locked to include the telephone set at this station with the bus conductors of the keys, thence over the respective contacts of'the key C to form a bridged circuit across the trunk conductors 49 and 50. The relay 51 is now operated through the telephone set at station B and in turn effects the operation of the relay 54. Separation of the contacts 64 of the relay 63 disconnects the transfer relay 67, and in turn the separation of the contacts 52 of the relay 51 disconnects the relay 63. The contacts 53 of the relay 51 reestablish the lefthand winding of the relay 58 on the batteries 2 and 3, by which the relay 58 is retained energized after the circuit through its right-hand Winding has been opened by the separation of the contact 68 of the relay 67. The relays 42 and 35 at station B, and the relays 51, 54, 58 and 60 of the trunk circuit, are now energized, and the circuits are thereby conditioned to permit a conversation to be carried on from the station B over the trunk circuit C. At the conclusion of the conversation and when the receiver at station B is replaced on the switchhook, the relay 60 is finally released to permit a disconnect signal to be displayed at the exchange, all in a manner similar to that already described for the dlsconnect operation from the station A.

From the foregoing description it Wlll be apparent that during the time a completed connection is present on the trunk circuit, the separated contacts 61 of the relay 60, in addition to the separated contacts 55 of the relay 54, disable the circuit of the conductor 48 through which it is necessary that the switching relay, as the rela 25 at station A, at another station must be initially operated In order to secure a connection with the trunk circuit. Therefore, during the time a call is being transferred and the relay'54 accordingly de'e'nergized, the contacts 61 of the relay 60 continue .the disabled condition of the conductor 48 to render the trunk circuit inaccessible to other stations that may attempt to secure a connection therewith by use of the trunk circuit key C The repeating coil 75 and the associated condensers of the trunk circuit C serve to prevent interference between the diiferent centralized current sources.

It will be understood that although a switching key may be retained fully depressed after a station or a trunk circuit call has been answered, this condition does not prevent or materially affect the talking properties of the connection.

The present invention is also arranged so that in addition to any station being able to answer a trunk call and to directly transfer such call to another station, it is also possible for any station in connection with the outside trunk to bring in another station and for all the parties so connected to converse together. By requiring a definite act on the part of the station in connection with the trunk line to bring in the second station, and a definite act by the party at the second station on answering, the secrecy of the system is preserved.

Let us assume that the party at station A is in connection with an outside party through the trunk line and desires to include station E in the conversation. The party at station A would press his transfer key C momentarily to hold the trunk line. The key B is next depressed to signal station B. On the party at station B answering, he would be informed that his presence was desired in a conversation between station A, B and the outside trunk line. B would thereupon press his trunk button C to intermediate position, and then depresshis key button C, thereby connecting his telephone set to the trunk line. The operation to this point is identical with the transfer of a trunk call .from station A to station B. The party at ter of station A, line 16, inner right-hand armature and front contact 28 of relay 25, contact 30, conductor 34 of station A, to line 34 of station B, which line extends from the transmitter and receiver of station B. Station A is thus placed in parallel with station B, and thus secures telephonic communication with the outside party over the trunk connection established with station B in the operation previously outlined.

Inasmuch as the connection for the outside trunk is now made at station B, the party at station A can withdraw from the conversation and hang up his receiver at any time without destroyin the connection existing between station and the outside party. As soon as the receiver is replaced at station A, the relay 10' falls ofi', resulting in the deenergization of relay 25, so that station A cannot again secure connection with station B or the trunk line while they remain busy, although station A is free to call idle stations in the usual manner. If, after station A has withdrawn from the conversation, the party at station B should desire to bring another station into the conversation, the operation previously described would be repeated for the desired station.

What is claimed is:

1. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, relays for each line circuit, centralized sources of current for said telephone system, and manually operated switching means for each line circuit adapted to control and cooperate with relay action on each one of two line circuits in completing a connection therebetween.

2. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, relays for each line circuit, centralized sources of current for said telephone system, manually operated switching means for each line circuit adapted to cooperate with said relays in completing a connection between two line circuits, and means operated by said relays to automatically alter the circuit connections of a line in order that the line will be accessible when idle and non-accessible when busy to other' lines which may attempt to secure a connection therewith.

3. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, relays for each line circuit, centralized sources of current for said telephone system, manually operated calling means for each line circuit, and means operated by said relays for normally disabling said manually operated calling means.

4:. In a telephone system, a plurality of line circuits, a telephone set. associated with each line circuit, relays for each line circuit, a battery current signal for each line circuit, centralized sources of current circuit for use.

6. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, relays for each line circuit, a battery current signal for each line circuit, calling means for each line circuit, centralized sources of current for said telephone system, and means controlled by said relays during the calling processes to permit said callingmeans to first seize'an idle line by setting up relay action thereon and second to condition said callin means to operate the signal associated wlth the line circuit seized.

7 In a telephone system, a plurality of line circuits, a telephone set for each line circuit, a battery current signal for each line circuit, relays for each line circuit, centralized sources of current for said telephone system, manually operated calling means for each line circuit adapted to cooperate with said relays in operating said signals, and means assocated with said calling means 00- operating with means associated with said re lays to condition the signal of a called line to produce a tone indication in the telephone receiverof the calling line.

8. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, relays for each line circuit, centralized sources of current for said telephone system, switching keys for each line circuit adapted to cooperate with action of said relays on each one of two lines in completing a connection therebetween, locking means operated by the keys for retaining said keys in an effective position, and circuit controlling contacts operated by said locking means to retain one of said relays energized during the time a connection is continued between two line circuits.

9. In a telephone system, a plurality of line circuits, a telephone set comprising a transmitter, receiver and hook switch associated with each line circuit, centralized sources of current for said telephone system, circuit connections individual to each of said sets through which each of said sets is connected to other line circuits seeking connection therewith, and means operated by removal of the telephone receiver of a set from its hook in initlating a call to open the circuit connection individual thereto to prevent connection with said telephone set by other line circuit-s seeking connection therewith.

10. In a telephone system, a plurality of line circuits, a telephone set com rlslng atransmitter, receiver and hook switch associated with each line circuit, centralized sources of current for said telephone system, circuit connections individual to each of said line circuits through which each of said line circuits is connected to other line circuits seeking connection therewith, and means operated by removal of the telephone receiver of a set from its book in initiating a call to 0 en the circuit connection individual to t e line circuit associated therewith to rotect such line circuit from seizure by ot er line circuits.

11. In a telephone system, a plurality of line circuits, a telephone set comprising a transmitter, receiver and hook switch associated with each 'line circuit, centralized sources of current for said telephone sys tem, circuit connections individual to each of said line circuits through which each of said line circuits is connected to other line circuits seeking connection therewith, and a relay for each line circuit operated by re moval of the telephone receiver of a-set from its hook in initiating a call to open the circuit connection individual to the line circuit associated therewith to protect such line circuit from seizure by other line circuits. I

12'. In a telephone system, a plurality of line circuits, a telephone set associated with each line circuit, centralized sources of current for said telephone system, circuit connections individual to each of said line circuits through which other line circuits seekin connection with such individual line circu1t obtain connection therewith, manually operated switching means for each line circuit, and means operated immediately upon the actuation of said switching means to, interrupt the circuit connectionindividual to the particular line circuit called to protect such particular line circuit from seizure by other of said other line circuits.

13. In a telephone system, a plurality of line circuits composed of two conductors only, a telephone set associated with each line circuit, a plurality of relays for each line circuit, centralized sources of current for said telephone system, test circuits for each of said line circuits extending over one of the line wires of the respective line circuit and conditioned by the operation of one of the relays of the respective line circuit called, another of said plurality of relays of each of said line circuits controlling the association of a calling line circuit with a called line circuit and included in said test circuit, and manually-operated switching means for each line circuit governing the first mentioned of said pluralitv of relays of its respective llne circuit to cond1t1on sald test circuit.

14. In a telephone system, a plurality of local lines, a telephone set associated with each local line, a. trunk line, means at each telephone set to effect connection with said trunk line, means to render a telephone set obtaining connection with said trunk line inaccessible to other lines, means for each of said telephone sets to secure connection with other of said sets, means to seize said other sets and render them inaccessible to other than the calling set, means whereby a trunk connection may be transferred from a telephone set connected with the trunk line to another set selected by the set first connected with the trunk line, and means whereby the trunk line is positivel held against seizure by sets other than t e one selected.

15..In a telephone system, a plurality of local lines, a telephone set associated with each local line, a, trunk line, means at each set to eflect connection with said trunk line, means to render a set obtaining connection with the trunk line free from seizure by other lines, means for each of said telephone sets to secure connection with other of said telephone sets, means toseize .said other sets and render them inaccessible to lines other than the callin line, and means to secure mutual telephomc connection between said trunk line and the telephone sets of a telephone sets, means to seize said other sets and render them inaccessible to lines other than the calling line, means to bring the telephone set of another local line into telephonic connection with the set of a local line connected to said trunk line, and means to connect the telephone set of said other local line telephonically with said trunk line to establish a mutual telephonic connection between said trunk line and the telephone sets of the connected local lines.

17. In a, telephone system, a, plurality of local lines, a telephone set associated with each local line, a trunk line, means at each set to effect connection with'said trunk line, means to render a set obtaining connection with said trunk line free from seizure by other lines, means for each of said telephone sets to secure connection with other of said telephone sets, means to seize said other sets and render them inaccessible to lines other than the calling line, means to bring the telephone set of another local line into telephonic connection with the-set of a local line connected to said trunk line, means to connect the telephone set of said other local line telephonically with said trunk line to establish a mutual telephonic connection between said trunk line and the telephone sets of the connected local lines, and means to enable the telephone set of one of said connected local lines being disconnected without dis- 10 In witness whereof I hereunto subscribe my name this 13th day of November, A. D. 15

CHARLES w. KECKLER. 

